Generally, position detectors and angle detectors are classified into different systems according to the detecting means employed, opto-electric system utilizing light, magnetic system utilizing magnetism, and contact system utilizing resistance value. Taking an encoder as an example in order to provide an explanation, with the opto-electric system infra-red light emitted from a light emitter such as a light emitting diode (infra-red LED) is alternately permitted and interrupted to pass by the combination of a fixed plate (phase plate) with peripheral slits permitting the transmission of the light and a plate (code wheel) formed with like peripheral slits and rotating together on a shaft whose motion is to be measured. The resulting intermittent light passing through the plates is received by a photodetector such as a photo-diode which produces a varying electric current, such current variations are then utilized for detection of the position (angle) and the rotational speed of the shaft.
Also included in the above opto-electric system is a detector of linear type operating on the same principle as the above rotary type for detecting the position of a linear motion. Each of the above two types is further divided into transmissive and reflective types depending upon the relative position of the light emitter with the photodetector with respect to the code wheel and phase plate. In the reflective type, the code wheel or the phase plate has its portion mirror-finished and both the light emitter and photodetector are positioned on the same side of the code wheel or the phase plate.
Further, there are incremental and absolute types depending upon the kind of output signal obtained. For example, with the absolute type the output is in the form of a four-bit signal in order to determine the absolute positions (in such a case 16-divided).
The light received by the photodetector causes changes in the electric current, which is in turn converted into changes in the voltage by a resistance inserted in series with the light emitter. The voltage change is such that the voltage level is gradually decreased as the slits in the phase plate become closed by the rotating code wheel and gradually increased as the slits in the phase plate become aligned with the slits in the code wheel to thereby produce a quasi-sine wave with an average value of direct current level above zero. The quasi-sine wave is fed into the Schmitt trigger circuit with a threshold level in the vicinity of the above average value in order to provide a rectangular waveform output with a 50% duty cycle.
The prior opto-electric encoders, however, will suffer from variations in the output from the photodetector when subjected to variations in the power supply voltage or to raised operating temperatures so that the duty cycle of the output wave will fluctuate with said threshold level remaining unchanged, resulting in the malfunction or counting error in a controller or a counter utilizing the output of the encoder.
Following is the explanation of the above in conjunction with the attached figures.
FIG. 1 shows a light receiving section employed in a prior opto-electric rotary encoder of transmissive type. The light receiving section includes a series combination of a light emitting diode LED and a resistor R inserted between the power source V.sub.cc and ground, a series combination of photodiode PD and load resistance R.sub.PD likewise inserted between the power source V.sub.CC and ground, and a set of code wheel 1 and phase plate 2 positioned between the light emitting diode LED and the photodiode PD.
The connection point between the photodiode PD and the load resistance R.sub.PD is connected to the inverting input of an operational amplifier OP, while a threshold voltage V.sub.TH is applied to the noninverting input of the operational amplifier OP. The output of the operational amplifier OP is in a positive feedback connection with the noninverting input through a resistor R.sub.0 so as to form the Schmitt trigger circuit.
FIGS. 2 to 5 are graphs showing the physical characteristics of the light emitting diode employed as a light emitter and the photodiode as a photodetector. The temperature vs. light intensity characteristics and the forward voltage vs. forward current characteristics of the light emitting diode are shown respectively in FIGS. 2 and 3. The temperature vs. photocurrent characteristics and the reverse voltage vs. photocurrent characteristics of the photodiode are shown respectively in FIGS. 4 and 5. As seen from these figures, when, for example, the ambient temperature is at 85.degree. , the luminous intensity falls to 65% of what it is at 25.degree. while the photocurrent increases to 120% of the value at 25.degree. C., whereby total efficacy is reduced to 78% of its level at 25.degree..
When on the other hand the power supply voltage is raised, the current flowing through the light emitting diode LED will increase correspondingly so as to likewise increase the light intensity thereof and therefore the output of the photodetector, although the voltage appeared across the load resistance R.sub.PD of the photodiode remains unchanged as seen from FIG. 5. At this occurrence, however, the increasing output of the photodetector will not be in direct proportion to the increasing power supply voltage due to the nonlinear relation between the forward voltage and the forward current of the light emitting diode LED.
Thus, the prior circuit having a fixed threshold voltage V.sub.TH fails to compensate for the above variations in the ambient temperature as well as power supply voltage and accordingly suffers from the variations in the output duty cycle, as shown in FIGS. 6 to 8.
The above will be now explained in the following sequence. Light emitted from the light emitting diode LED upon flowing of the current therethrough will pass through the rotating code wheel 1 and the phase plate 2 to reach the photodiode PD when the slits of the both are in aligngment with one another. The photodiode PD upon receiving the light causes a current flow between the power source V.sub.CC and ground so as to produce a voltage across the load resistance R.sub.PD, which voltage is then fed to the inverting input of the operational amplifier OP forming the Schmitt trigger circuit. Since the slits of one of the code wheel and the phase plate are gradually opened and then closed as the code wheel rotates, the change of the voltage fed to the operational amplifier is such that a quasi-sine wave having an average value of direct current level above zero is generated. The curves i, ii, and iii of FIG. 6 respectively show possible quasi-sing waveforms, in which the curve i in solid line is for the waveform with an average potential in normal condition, the curve ii in broken line is for the waveform with a greater average potential, and the line iii in dotted line is for the waveform with a less average potential. The quasi-sine waveform, when fed to the Schmitt trigger circuit having the fixed threshold voltage V.sub.TH determined by the average potential in the normal condition, provides a rectangular waveform output as shown in FIG. 7 for the condition where the average potential becomes less than that in the normal condition by being subjected to the changing ambient temperatures, and provides a rectangular waveform output as shown in FIG. 8 for the condition where the average potential becomes greater than that in the normal condition due to the raising power supply voltage, whereby an uniform duty cycle cannot be expected. The present invention has been achieved for the purpose of eliminating the above problems associated with the prior circuit and therefore providing an improved opto-electric position detector circuit.